Image forming apparatus using electrophotographic systems, such as copying machines, printers, facsimile machines or complex machines combining each function of those, have used various development systems such as a binary development system, a monocomponent development system, a hybrid development system, etc. The binary development system uses a toner and a magnetic carrier while the monocomponent development system uses only a toner as a developer. The hybrid development system uses a binary developer using a magnetic carrier for charging a non-magnetic toner. In the hybrid development system, only the charged toner is retained on a developing roller and brought into flight to or contact with an electrostatic latent image so as to develop the latent image.
The binary development system is superior in the electrostatic property of the toner due to the carrier and capable of extending the life of the apparatus. In addition, the binary development system is advantageous in equalizing a solid image. However, the binary development system has drawbacks as follows. That is, (1) the binary development system requires a large and complicated development device; (2) toner flying or carrier drawing may occur; and (3) the quality of an image may be shifted in accordance with the durability of the carrier.
The monocomponent development system can use a compact development device and is superior in dot reproducibility. However, the monocomponent development system has drawbacks as follows. That is, (1) due to the durability lowered by the deterioration of a developing roller, the development device must be replaced to increase the expenses of supplies; and (2) selective development may occur. On the other hand, the hybrid development system is a system which combines the binary development system and the monocomponent development system. The hybrid development system is superior in dot reproducibility and capable of extending the life of the apparatus and forming an image at a high speed.
One of the background-art techniques as to hybrid development is disclosed in U.S. Pat. No. 3,866,574. According to the technique, a thin layer is formed out of a non-magnetic toner on a donor roller (developing roller) placed in non-contact with an image carrying member, and the toner is made to fly to a latent image on the image carrying member by an AC electric field. U.S. Pat. No. 3,929,098 discloses a development device in which a developer is carried to a donor roller by a magnetic roller, and a toner is transferred onto the donor roller so as to form a toner layer.
According to these techniques, a thin layer can be formed on a donor roller by use of a binary developer. It is, however, difficult to separate a toner on the donor roller when the toner is highly charged. Thus, a high AC electric field is required. The electric field disturbs a toner layer on an image carrying member so as to cause trouble for color layering or the like. As a solution to the trouble, JP-A-3-113474 discloses a so-called powder cloud development method in which an auxiliary electrode composed of wires is placed between a donor roller and an image carrying member, and a weak AC current is applied to the auxiliary electrode so as to prevent a developed toner from being disturbed.
These background-art techniques are based on non-contact development using a hybrid development system. The non-contact development has problems as follows. That is, it is necessary to keep a gap between a photoconductor and a developing roller on the order of several tens of micrometers and with a high precision. In addition, an AC current has to be applied to the developing roller. As a result, the configuration of a development device is complicated, and fine dot images or gradations cannot be reproduced satisfactorily.
On the other hand, contact development apparatus using a hybrid development system have been proposed as follows. JP-A-55-77764 discloses a method of contact development using a sponge roller as a developing roller. Japanese Patent No. 3356948 or Japanese Patent No. 3404713 discloses a method of contact development using a developing roller composed of an elastic roller fitted into a metal sleeve.
In some electrophotographic image forming apparatus, a photoconductor whose substrate is composed of a metal drum is used as a member for forming an electrostatic latent image. Generally in such an image forming apparatus, a developing roller composed of an elastic body such as rubber or sponge has to be pressed onto the photoconductor at a constant pressure in order to secure a developing range where the developing roller can come in contact with the photoconductor so as to carry out stable development. However, the developing roller composed of rubber, sponge or the like has a problem as follows. That is, the developing roller is vulnerable to sliding on a control member such as a blade, or vulnerable to abrasion with a binary developer using a magnetic carrier.
The aforementioned Japanese Patent No. 335694 and Japanese Patent No. 3404713 are solutions to the problem. However, each solution has a problem as follows. That is, the photoconductor is rubbed by the metal sleeve so that the surface of the photoconductor may be easily damaged, or the metal sleeve is separated from the elastic roller so that a long life cannot be given to the developing roller.
Patent Document 1: U.S. Pat. No. 3,866,574
Patent Document 2: U.S. Pat. No. 3,929,098
Patent Document 3: JP-A-3-113474
Patent Document 4: JP-A-55-77764
Patent Document 5: Japanese Patent No. 3356948
Patent Document 6: Japanese Patent No. 3404713
As described above, in order to achieve contact development in a background-art development device using a hybrid development system, a developing roller which will come in contact with a photoconductor has to be composed of an elastic body. However, in a process where the developing roller composed of an elastic body is rubbed by a binary developer composed of a magnetic carrier and a toner so that a predetermined quantity of the toner is applied to the surface of the developing roller, the surface of the developing roller is abraded and roughed by the binary developer. Thus, the toner is applied to the surface of developing roller unevenly. As a result, the life of the developing roller is apt to be shortened.
A method using a developing roller in which a metal sleeve is fitted to the surface of an elastic roller also has problems as follows. That is, a photoconductor is rubbed by the metal sleeve so that the surface of the photoconductor is easily damaged, or the metal sleeve is separated from the elastic roller. It is therefore difficult to elongate the life of the developing roller.